The actual meaning for living fossil, is any creature that has been on earth for many millions of years and has not changed much, if at all, during that time. In this respect one would expect to find tuataras, crocodiles, sharks, coelocanths and cockroaches in the group, but it turns out that these animals have had varying degrees of differentiation as well. Stating that a tuatara of today is the exact same as one 140 mya would be false. Studies on sphenodont species found in fissure deposits have shown that tuataras are actually a specialized form of sphenodont and not the "original blueprint".
In truth the only real living fossils would be the dinosaurs in Jurassic Parktm but that, of course, is science fiction :)